In the setting of severe trauma and sepsis, a breakdown in gut barrier function has been termed the "motor of multi-system organ failure", but the underlying pathophysiology is unknown. This proposal is based upon a novel hypothesis that the loss of intestinal alkaline phosphatase (IAP) expression underlies the impairment in gut mucosal defense that occurs in critically ill patients. Although IAP is a well-known marker for crypt-villus differentiation, its physiological role has not been elucidated. We now believe that IAP represents a previously unrecognized gut defense mechanism that is designed to protect the organism from bacteria and toxins. The broad goals of this proposal are to (1) define the mechanisms by which IAP protects the organism from the luminal environment, (2) prove that this newly recognized component of the gut mucosal defense system actually exists in vivo, and (3) lay the groundwork for IAP-based therapies. In Aim #1, in vitro model systems will be used to examine the mechanisms by which IAP protects the organism from luminal bacteria (e.g., Salmonella) and toxins (LPS). WT and IAP-expressing cells will be employed to identify the cellular targets for IAP, delineating its mechanism of action in protecting the host cell. In Aim #2, in vivo model systems will be employed in WT and IAP KO mice in order to assess its functional role in regard to gut mucosal defense. Small intestinal ischemia and luminal Salmonella/LPS will be used to test the ability of the intestinal barrier to protect the host from infection, sepsis, and end organ damage. Aim #3 is designed to establish the efficacy of utilizing IAP as a target in the context of a variety of disease states. The two models of sepsis/injury will be applied in the contexts of two susceptibility conditions in which IAP expression is lost, starvation and hypothyroidism. These studies in IAP KO and transgenic mice will allow us to determine the role that IAP plays in the gut barrier dysfunction that accompanies clinically relevant disease states. Taken together, these studies will elucidate the functions of IAP and its mechanisms of action, and are designed to provide the "proof of principle" for IAP-targeted therapies that could be used in the clinical setting to maintain gut mucosal defense function. In lay terms, we will explore a previously unrecognized mechanism that the organism has developed to protect itself from harmful substances within the gastrointestinal tract, laying the groundwork for treatments that will aid critically-ill patients. The proposed studies will explore a previously unrecognized gut mucosal defense mechanism involving the intestinal brush border enzyme, intestinal alkaline phosphatase (IAP). The studies will elucidate the functions of IAP and its mechanisms of action, and are designed to provide the "proof of principle" for IAP- targeted therapies that could be used in the clinical setting to maintain gut mucosal defense function.